Energy absorbing materials and structures are used in a number of applications including vehicles, ballistic armor and helmets, sporting equipment and clothing. Such materials will shunt, convert or dissipate energy via viscosity, friction, visco-elasticity or plasticity. Open or closed cell foams, fibrous materials, springs or piston-cylinder arrangements are generally used as energy-absorbing structures.
The energy absorption capacity of the material is its defining characteristic. If the absorption capacity is too low, the material “bottoms out,” providing no additional protection. If the absorption is too high, the force exerted may exceed a critical level and cause damage or injury. Thus, a more compliant material generates low forces and is comfortable, but absorbs very little energy. There is thus a trade-off need to achieve the optimal stiffness property of a given material. In addition, while bulkier, thicker materials will absorb more energy, greater bulk may conflict with design demands for slim, narrow structures.
Energy absorption in occupant protection components for vehicle and passenger safety is a particularly important concern. As an example, the National Highway Traffic Safety Administration (NHTSA) has estimated that there were 805,851 occupants with whiplash injuries annually between 1988 and 1996 in the United States, resulting in a total annual cost of $5.2 billion. Whiplash associated disorders are influenced mainly by properties and positioning of seat and head-restraints, as reported by Jakobsson, Lundell, et al. in “WHIPS—Volvo's Whiplash Protection Study.” Accid. Anal. Prev. 32(2): 307-19 (2000). The number and extent of injuries can be reduced by maximizing the amount of energy absorption, by minimizing the occupant acceleration or by reducing the relative movement between the head and the torso. This, however, indicates a possible conflict in the stiffness property of head-restraints and seat back-rests, since a more energy absorbing structure is rigid under everyday shock conditions leading to discomfort.
Similarly, in side crashes, NHTSA simulation studies have shown that structural stiffness and energy management through padding in doors or pillar trims can significantly reduce chest, head or pelvic injuries. Further, in compliance with new legislation, vehicle components must meet impact safety standards. The European Experimental Vehicles Committee Working Group (EEVC WG 17) and European New Car Assessment Program (EURO NCAP) require vehicle designs (exterior parts) to minimize pedestrian injuries due to impact. The coincidence of the adult upper leg impact zone with the child head impact zone indicates again a “conflict of stiffness” problem as described by Courtney and Oyadiji in “Preliminary investigations into the mechanical properties of a novel shock absorbing elastomeric composite,” Journal of Materials Processing Technology 119 (1-3): 379-386 (2001). The Federal Motor Vehicle Safety Standard (FMVSS 201/202) further specifies special requirements for the interior parts of the vehicle such as A/B/C pillar trims, head-liners, and knee and side impact foam parts. These impact parts are required to satisfy a number of energy absorption criteria under different impact conditions to reduce injuries to different passengers during a collision. Energy absorbing materials implemented previously usually meet either the child passenger impact criteria or the adult passenger impact criteria but not both sets of criteria.
Fluid-filled cells and absorbent matrices have been used to implement energy absorbing devices. For example, U.S. Pat. Nos. 5,915,819, 5,564,535 and 3,672,657 disclose structures made of a series of fluid-filled cells or reservoirs, wherein energy absorption is achieved by restricting the fluid-flow through orifices or in-between cells and reservoirs. Similarly, International Publication No. WO 99/49236 describes an energy absorbing material wherein the fluid-filled cells are permeable. Energy absorbing pads or bladders which form fluid-filled envelopes or compartments have been employed; for example, U.S. Pat. No. 5,545,128 utilizes a compartments filled with a shear-thickening fluid to form pads for bone fracture prevention. U.S. Pat. No. 6,202,806 describes motion control devices which utilize an absorbent matrix to hold a Theological fluid that is subjected to a magnetic field, the matrix being positioned between two moving members and acting in a shear mode to control the relative sliding, linear or rotary motion of the members.